Please refer to FIG. 6A, which is the sectional schematic view of a surface acoustic wave resonator of conventional technology. A surface acoustic wave resonator 60 comprises two interlocking comb-shaped electrodes 611, 612 of an interdigital transducer 610 and two grating reflectors 613, 614. The two interlocking comb-shaped electrodes 611, 612 of the interdigital transducer 610 and the two grating reflectors 613, 614 are formed on a piezoelectric substrate. The two grating reflectors 613, 614 are formed respectively at two sides of the two interlocking comb-shaped electrodes 611, 612 of the interdigital transducer 610. However, the interdigital transducer 610 of the surface acoustic wave resonator 60 is sensitive to thermal variation. A thermal variation will cause a variation of the resonance frequency of the surface acoustic wave resonator 60. Also a thermal variation will cause a variation of an equivalent parallel capacitance of the interdigital transducer 610 of the surface acoustic wave resonator 60. Please also refer to FIG. 6B, which is the sectional schematic view of an acoustic wave resonance structure of a bulk acoustic wave resonator of conventional technology. In conventional technology, a bulk acoustic wave resonator (BAW) or a thin film bulk acoustic wave resonator (FBAR) has an acoustic wave resonance structure 64. The acoustic wave resonance structure 64 comprises a bottom electrode 61, a piezoelectric layer 62 and a top electrode 63. The piezoelectric layer 62 is formed on the bottom electrode 61. The top electrode 63 is formed on the piezoelectric layer 62. However, the acoustic wave resonance structure 64 of the bulk acoustic wave resonator (or the thin film bulk acoustic wave resonator) is sensitive to thermal variation. A thermal variation will cause a variation of the resonance frequency of the acoustic wave resonance structure 64 of the bulk acoustic wave resonator (or the thin film bulk acoustic wave resonator). Also a thermal variation will cause a variation of an equivalent parallel capacitance of the acoustic wave resonance structure 64 of the bulk acoustic wave resonator (or the thin film bulk acoustic wave resonator). Therefore, a thermal sensor and an active thermal compensating circuit are needed for the surface acoustic wave resonator 60 (SAW), the bulk acoustic wave resonator (BAW) and the thin film bulk acoustic wave resonator (FBAR). And also a thermal compensation method is needed.
Please refer to FIG. 6C, which is the sectional schematic view of a thermal sensitive resistance sensor of conventional technology. A thermal sensitive resistance sensor 65 has a meandered-shape. In conventional technology, the thermal sensitive resistance sensor 65 is positioned near the surface acoustic wave resonator 60, the bulk acoustic wave resonator or the thin film bulk acoustic wave resonator for sensing a thermal variation. However, it costs further effort to form a thermal sensor near the acoustic wave resonator.
A conventional buck DC-DC converter circuit is usually used for stepping down voltage from its input to its output. A conventional boost DC-DC converter circuit is usually used for stepping up voltage from its input to its output. But no one has applied the conventional buck DC-DC converter circuit or the conventional boost DC-DC converter circuit as a thermal sensor circuit, especially a thermal sensor circuit for a thermal sensor which senses a thermal variation correlated to a capacitance variation of the thermal sensor.
Accordingly, the present invention has developed a new design which may avoid the above mentioned drawbacks, may significantly enhance the performance of the devices and may take into account economic considerations. Therefore, the present invention then has been invented.